Attempts are being made at present in the manufacture of paper to produce paper of ever higher quality. The demand for high quality paper has increased considerably in recent times. Among other things, good printing properties are required from paper so that the importance of coating and surface sizing of paper has increased remarkably.
In the production of paper of higher quality, it is necessary to have ever more accurate information with respect to the various parts of the manufacture so that necessary adjustments can always be carried out when any disturbance in the paper quality occurs. In the case of surface sizing and coating of paper, such information is needed, for example, with respect to the thickness of the coating or the size layer at a stage as early as possible in order to permit immediate reaction to disturbances which are occurring.
Since, in the coating of paper, additives are frequently used for the purpose of improving the properties of paper, which additives contain, for example, titanium dioxide or calcium carbonate, which are fluorescent substances, the measurement of coating quantity based on X-ray fluorescence has proved to be effective. In the prior art, the measurements have, however, been carried out directly from the paper to which the coating agents have already been added, or on paper which has already been surface-sized. One arrangement of this type is described in Finnish Pat. No. 70,750. However, due to the fact that the measurement is carried out directly from the coated paper, the method involves a number of interfering factors, as a consequence of which a sufficiently accurate measurement of coating quantity cannot be achieved. Still further, since the measurement takes place directly from the paper, it is not possible to react sufficiently quickly to effect necessary corrections.